docs: def get_aad_content_string(content_type, is_final_frame):
      """Prepares the appropriate Body AAD Value for a message body.
  
      :param content_type: Defines the type of content for which to prepare AAD String
      :type content_type: aws_encryption_sdk.identifiers.ContentType
      :param bool is_final_frame: Boolean stating whether this is the final frame in a body
      :returns: Appropriate AAD Content String
      :rtype: bytes
      :raises UnknownIdentityError: if unknown content type
      """
      if content_type == ContentType.NO_FRAMING:
          aad_content_string = ContentAADString.NON_FRAMED_STRING_ID
      elif content_type == ContentType.FRAMED_DATA:
          if is_final_frame:
              aad_content_string = ContentAADString.FINAL_FRAME_STRING_ID
          else:
              aad_content_string = ContentAADString.FRAME_STRING_ID
      else:
          raise UnknownIdentityError("Unhandled content type")
      return aad_content_string